There has been an eager demand for reduction in size and weight of portable electronic equipment, and the realization relies heavily on improvement of battery performance. To meet the demand, development and improvement of a variety of batteries have been proceeding. Battery characteristics expected to be improved include increases in voltage, energy density, resistance to high load, freedom of shape, and safety. Of currently available batteries, lithium ion batteries are the most promising secondary batteries for realizing a high voltage, a high energy density, and excellent resistance to high load and have been and will be given improvements.
A lithium ion secondary battery mainly comprises a positive electrode, a negative electrode, and an ion conducting layer interposed between the electrodes. The lithium ion secondary batteries that have been put to practical use employ a positive plate prepared by applying to an aluminum current collector a mixture comprising a powdered active material, such as a lithium-cobalt complex oxide, a powdered electron conductor, and a binder resin; a negative plate prepared by applying to a copper current collector a mixture of a powdered carbonaceous active material and a binder resin; and an ion conducting layer made of a porous film of polyethylene, polypropylene, etc. filled with a nonaqueous solvent containing lithium ions.
FIG. 5 schematically illustrates a cross section of a conventional cylindrical lithium ion secondary battery disclosed in JP-A-8-83608. In FIG. 5 reference numeral 1 indicates a battery case made of stainless steel, etc. which also serves as a negative electrode terminal, and numeral 2 an electrode body put into the case 1. The electrode body 2 has a roll form composed of a positive electrode 3 and a negative electrode 5 having a separator 4 therebetween. In order for the electrode body 2 to maintain electrical connections among the positive electrode 3, the separator 4, and the negative electrode 5, it is necessary to apply pressure thereto from outside. For this purpose, the electrode body 2 is put into a firm case 1 to apply pressure for maintaining all the planar contacts. In the case of rectangular batteries, an external pressing force is imposed to a bundle of strip electrodes by, for example, putting the bundle in a rectangular metal case.
That is, a contact between a positive electrode and a negative electrode in commercially available lithium ion secondary batteries has been made by using a firm housing made of metal, etc. Without such a housing, the electrodes would be separated at their interface, and the battery characteristics would be deteriorated due to difficulty in maintaining electrical connections. However, occupying a large proportion in the total weight and volume of a battery, the housing causes reduction in energy density of the battery, Moreover, being rigid, it imposes limitation on battery shape, making it difficult to make a battery of arbitrary shape.
Under such circumstances, development of lithium ion secondary batteries which do not require a firm housing has been proceeding, aiming at reductions in weight and thickness. The key to development of batteries requiring no housing is how to maintain an electrical connection between each of a positive electrode and a negative electrode and an ion conducting layer (i.e., separator) interposed therebetween without adding an outer force.
Joining means requiring no outer force that have been proposed to date include a structure in which electrodes (a positive and a negative electrode) are joined with a liquid adhesive mixture (gel electrolyte) as disclosed in U.S. Pat. No. 5,460,904 and a structure in which an active material is bound with an electron conducting polymer to form a positive and a negative electrode, and the electrodes are joined via a polyelectrolyte as disclosed in U.S. Pat. No. 5,437,692.
Conventional lithium ion secondary batteries having the above-mentioned structures have their several problems. Those in which a firm case is used for ensuring intimate contacts and electrical connections between electrodes and a separator have the problem that the case which does not participate in electricity generation has a large proportion in the total volume or weight of a battery, which is disadvantageous for production of batteries having a high energy density. On the other hand, the structure in which electrodes are joined with a liquid adhesive mixture needs a complicated production process and hardly shows sufficient adhesive strength for securing improved strength as a battery. The structure in which electrodes are joined with a polyelectrolyte is disadvantageous in that the polyelectrolyte layer should have a sufficient thickness for security, i.e., enough to prevent internal shortage between electrodes, failing to provide a sufficiently thin battery; a solid electrolyte is insufficient to join an electrolyte layer and an electrode active material, making it difficult to improve battery characteristics such as charge and discharge efficiency; and the production process is complicated, resulting in an increase of cost.
Efficiency in intercalation and disintercalation of lithium ions by active materials occurring on charging and discharging of a battery is an important factor decisive of the charge and discharge efficiency of a battery. In a battery of ordinary structure, because mobility of lithium ions is equal throughout an electrolytic solution, there is a problem that intercalation and disintercalation of lithium ions take place preferentially in the portion of the active material layer in the vicinity of the electrode surface, i.e., in the vicinity of the separator so that the active material in the inside of the electrode is not made effective use of. As a result, desired charge and discharge characteristics are hard to obtain.
Hence, in order to obtain a practical thin type lithium ion battery, it is required to develop a battery structure that exhibits satisfactory battery characteristics such as charge and discharge characteristics while easily securing safety and strength as a battery. That is, it is necessary that a separator is provided between electrodes for safety and that the separator and the electrodes are joined with sufficient strength and in such a manner that secures satisfactory battery characteristics.
In order to solve these problems, the inventors of the present invention have conducted extensive study on a favorable method for adhering a separator to a positive and a negative electrode. The present invention has been reached as a result. Accordingly, an object of the present invention is to provide a compact and stable lithium ion secondary battery in which a positive electrode, a negative electrode, and a separator are brought into firm and intimate contact without using a firm battery case, which can have an increased energy density, a reduced thickness, and a plurality of electrode laminates in an arbitrary shape, exhibits excellent charge and discharge characteristics, and has a large battery capacity.